pckbd: set bits 2-3-6-7 of the output port by default

pckbd: set bits 2-3-6-7 of the output port by default

OSes typically write 0xdd/0xdf to turn the A20 line off and on.  This
has bits 2-3-6-7 on, so that the output port subsection is migrated.
Change the reset value and migration default to include those four
bits, thus avoiding that the subsection is migrated.

This strictly speaking changes guest ABI, but the long time during which
we have not migrated the value means that the guests really do not care
much; so the change is for all machine types.

Reported-by: Igor Mammedov <imammedo@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>